This application is based on Japanese Patent Application No. 2000-277614 filed on Sep. 13, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical waveguide, and more particularly to an optical waveguide having waveguide and clad layers made of fluorinated polyimide.
2. Description of the Prior Art
Polyimide has been widely used as material for interlayer isolation films in semiconductor devices and for other uses, but has rarely been used as material for waveguides because of its poor optical transparency. It has been found, however, that substituting fluorine atoms for about 30% or more of hydrogen atoms in polyimide markedly improves its optical transparency toward light in the visible range. Moreover, in the first place, polyimide is excellent in heat resistance, and can be soldered on the surface of a laser or photoreceptor device. For these desirable properties, polyimide has recently been in increasingly wide use as optical material.
However, a fluorinated polyimide layer (i.e. a layer containing fluorinated polyimide as its main ingredient) is poor in adherence to a substrate, and therefore, when an optical waveguide is diced into chips, the fluorinated polyimide layer often peels off the substrate at the edges of the chips. In particular, in high-temperature, high-humidity environments, such peeling-off is so striking that the fluorinated polyimide layer sometimes separates completely from the substrate. For this reason, it has been proposed to prevent a fluorinated polyimide layer from peeling off a substrate by laying a coupling agent layer between the fluorinated polyimide layer and the substrate, or by increasing the surface roughness of the substrate. These treatments, however, have been unable to offer a satisfactory solution to date.
An object of the present invention is to provide an optical waveguide having a fluorinated polyimide layer with improved adherence to a substrate so that the fluorinated polyimide layer is less prone to peel off the substrate.
To achieve the above object, according to one aspect of the present invention, an optical waveguide is provided with: a substrate; a primer layer formed on the substrate; and a waveguide layer formed on the primer layer. Here, the primer layer contains as the main ingredient thereof polyimide with a fluorine substitution rate of 10% or lower (including 0%) and the waveguide layer contains as the main ingredient thereof polyimide with a fluorine substitution rate of 30% or higher.
This arrangement improves the adherence of the waveguide layer to the substrate, and thereby prevents the waveguide layer from peeling off the substrate. In the present specification, the fluorine substitution rate denotes the rate at which fluorine atoms are substituted for hydrogen atoms in polyimide.
According to another aspect of the present invention, an optical waveguide is provided with: a substrate; a primer layer formed on the substrate; and a clad layer formed on the primer layer and having a waveguide layer. Here, the primer layer contains as the main ingredient thereof polyimide with a fluorine substitution rate of 10% or lower (including 0%) and the waveguide layer contains as the main ingredient thereof polyimide with a fluorine substitution rate of 30% or higher.
This arrangement improves the adherence of the clad layer to the substrate, and thereby prevents the clad layer from peeling off the substrate.
According to another aspect of the present invention, a method of manufacturing an optical waveguide includes: a step of forming a primer layer on a substrate; a step of forming a lower clad layer on the primer layer; and a step of forming a waveguide layer selectively on the lower clad layer. Here, the primer layer contains as the main ingredient thereof polyimide with a fluorine substitution rate of 10% or lower (including 0%) and the lower clad layer and the waveguide layer contain as the main ingredient thereof polyimide with a fluorine substitution rate of 30% or higher.